1. Field of the Invention
The present invention pertains generally to the field of precision time transfer, more particularly to the field of time transfer by electromagnetic signals.
2. Background of the Invention
Accurate time, traceable to a standard, has been a challenge and objective through the ages. In each age, as timekeeping becomes more accurate, more and more uses are found for the more accurate time keeping. In early times, time keeping was only used to arrange meetings to minimize lost productivity as people were assembled. As time keeping advanced, time could be used to help determine longitude for seagoing ships and for recording and measuring astronomical events. As civilization advanced, time zones were established to simplify train scheduling and coordinate commerce at multiple locations now linked by high speed transportation and communications. In modern times, precision time accurate to milliseconds or better is used to coordinate events and measurements. For example, in a missile test, the precision comparison of the timing of events and measurements on board the missile with ground based radar and optical observation is critical to the evaluation of missile performance and the determination of faults and errors needed for system development.
Precision timing also finds use in coordination of data from widely separated radio telescopes, enabling much finer resolution and image detail. Before GPS, the National Institute of Standards and Technology (NIST) (then, NBS) operated WWV, WWVL, short wave and long wave time and frequency standards that are still operating and still in use by many systems. Precision time has also been transmitted over TV network systems using horizontal sync and color subcarrier signals. Recently, GPS has been developed to provide accurate time transfer as well as position measurement. Basic accuracy may be 100 ns or better with differential capability to 1 ns or better for the high performance systems.
With the availability of higher precision time, applications are finding more uses and demanding greater accuracy in time measurements and timing comparisions. Sporting events, for years, were happy with tenth of a second resolution in stop watches. Then hundredths of a second became available. Now some events are decided by milliseconds and there is talk of 100 microseconds.
Thus, there exists a present and future need for ever more precise and accurate time transfer and time measurement by simpler and more economical devices that can serve consumer and commercial applications.